What feedback shall I leave?

Background: Listed a pair of Croc wellies on Ebay. Title was Croc wellies sz 8 9. In item categories I put size 8.

On the day that I'd posted the wellies, I received this email:

Was just wondering if you had sent these wellies yet? I have just received another pair in another colour, size 8/9, but they are a US size 8/9, not UK and are therefore too small.

If yours are a UK size 8 that's fine but if not I would like to request to cancel the order and receive a refund as they will be no good to us.

I replied with:

Sorry, I sent the wellies earlier on today before I saw your message. These wellies were purchased in the UK, but I'm not sure if they are a UK or US size. My little boy is now in Clarks size 10 shoes, and he has just outgrown them if that helps at all?

The buyer then left neutral feedback with the following comment: As shown but very small for size

I replied with: They were listed accurately. Unsure why this warrants a neutral feedback.

Now I can completely understand why she isn't happy with the purchase if they don't fit her little one, but my DS outgrew these in August, and the same weekend we had his feet measured for his school shoes (whilst still wearing these wellies in the shop as we were on our way back from a camping weekend!), and he was a 10 in Clarks. Which leads me to think that the sizing on the boots is about right at 8-9.

What feedback would you leave? Sorely tempted to leave a negative or neutral, but can kind of see her point if she's disappointed. WWYD?

you can't leave a neg or a neut for a buyer.you have replied to her FB already which more or less shows her to be the unreasonable one. Nothing more you can do now. Best not to leave her any FB at all as a positive will only boost her FB number